The use of renewable energy source (RES) systems, such as photovoltaic (PV) systems, rectified wind turbine systems, fuel cell systems and ultracapacitor systems, which incorporate battery backup systems, is expected to increase as the Feed-in Tariff (FiT) rates are set to decline and conventional electricity prices are projected to rise. Additionally, the use of renewable energy sources with battery backup systems is expected to increase as a result of the ability of the integrated systems to mitigate the high penetration issue into the grid.
With Feed-in Tariff rates decreasing, users are moving away from buying renewable energy systems as an investment in order to capitalize on attractive government incentives, and are instead moving toward buying systems that will save energy and money, by generating their own electricity and reducing their reliance on electricity provided by utility companies. Adding a battery backup system to the renewable energy system is seen as an effective way of retaining the less expensive renewable energy electricity that users are generating, allowing them to use their renewable energy generated electricity later in the day to avoid paying high prices for the utility electricity, especially during peak demand time.
As currently known in the art, renewable energy system converters and battery backup converters are manufactured as separate power converters, and as such, the complexity of power management and system cost is increased due to the requirement of the additional battery backup converter.
As shown in FIG. 1, in a conventional system, the utility grid 105 interfaces with a DC bus 115 through a DC/AC converter 140. In addition the system may include, a local system 100 incorporating a battery back system (Energy Storage System, ESS) 125 coupled to the DC bus 115 through a DC-DC converter 130, a renewable energy system 135 coupled to the DC bus 115 through a AC, DC-DC converter 120 and a DC Load 110 coupled to the DC Bus 115. In the conventional system, the battery backup system 125 and the renewable energy system 135 are coupled to the DC bus 115 by two independent power converters, one power converter 130 for the battery backup system 125 and a separate power converter 120 for the renewable energy system 135. However, implementing two independent power converters 120, 130 requires the use of a complex power management system for switching between the battery backup system 125 and the renewable energy system 135 and for the required charging and discharging of the batteries.
Multi-port DC-DC converters are known in the art as an attractive solution that is suitable for integrating the renewable energy system and the battery backup system, providing the advantages of high efficiency, high power density and cost-effectiveness. In particular, magnetic coupled three-port DC-DC converters have been proposed to interface with the renewable energy system and the battery backup system. However, DC-DC converters known in the art are not capable of charging batteries from a common DC bus side, due to the unidirectional power flow between the primary and secondary side of the high frequency transformers of the DC-DC converter. Moreover, it is desirable to decouple the power control of the renewable energy system and the battery back-up system so that a decoupled controller can be designed. Additionally, the systems known in the prior art also result in an undesirable circulating current between the three ports of the DC-DC converter, which results in a decrease in the system efficiency.
Accordingly, what is needed in the art is a distributed renewable energy system having an integrated battery backup system and an intelligent energy management system that can control the power flow among the energy sources, batteries, load and utility grid in order to achieve the most economical and efficient electric supply.